Vapor Permeable, UV Resistant Air Barrier Sheet Membrane

[Ted S.]

I need a good product for a insulated basement; the current installation is a damproofed concrete below grade foundation wall, wood studs and fiberglass batt insulation wall with fire retardant poly.

I'd like to find a vapor permeable replacement for the poly. There are some good houswraps, but most all have UV limitations.

[worthy]

Membrain.

[Martin Holladay]

Ted,
1. What type of UV exposure does a basement wall get?

2. Your real problem isn't choosing a vapor-permeable way to cover the fiberglass batts -- your real problem is the fiberglass batts, which don't belong in a basement. They belong in the dumpster. Then you can install rigid foam or closed-cell spray polyurethane foam.

[Ted S.]

Thanks for the response fellas.

Membrain is an interesting product, I'd have to look at it in detail. So if the concrete is damp, is that producing conditions that will increase it's VP? I guess what I would rather see is a 100% vapor permeable product. I am just trying to satisfy an air barrier requirement.

Martin,

I understand what you are saying, but this developer has 500 existing units with no problems in any of the units which have finished basements. Cost is an issue and I don't think I could ever get him to install CCSPF. Although he did get it in above grade walls for the last 15 units, but I think that is about to end.

I am on record in regard to moisture problems, I just want to try and make the best out of this situation. I have data loggers rigged up in several units. So far no problems.

In terms of UV degradation, I have no idea if that is really an issue, but the building inspector has called it out and technically there is no way to fight it as there are windows, so I'd rather just find something that is UV stable.

[David Meiland]

Ted, is the existing poly against the concrete, or on the face of the studs? And why is an air barrier needed given that the concrete is probably doing a pretty good job of it?

[worthy]

So I take it the inspector says that sunlight through the windows will degrade the poly, so he wants it removed. That's a new one for me! You might be able to challenge that at a higher level of the inspection chain.

Membrain is similarly susceptible to UV damage and should not be used where it is exposed to sunlight. (See Specification Sheet.) In which case, you're left with having to fully finish the walls as is or installing an insulation better suited to below grade use such as EPS, XPS or SPF and covering it with an acceptable thermal barrier.

no problems in any of the units which have finished basements.

A mould test might prove that a mistaken belief. OTOH, I've lived in my own homes with similarly finished basements and noticed no obvious problems. But I dehumidify 24/7/365. (And I don't insulate that way anymore!)

Bad building science accepted by the building authority--poly over fg in a basement--might be the better economic route for the developer. But if the walls fail within a short period, there could be a legal mess to come. Mould suits have subsided partly because builders have become more pro-active in the use of sound construction practices. But your client can always throw the dice.

See pic. [Attached photo by Joseph Lstiburek, Building Science Corporation http://www.buildingscienceconsulting...lstiburek.aspx

[Ted S.]

Ted, is the existing poly against the concrete, or on the face of the studs? And why is an air barrier needed given that the concrete is probably doing a pretty good job of it?

Because the ENERGY STAR Qualified Homes, Version 3 (Rev. 03) Thermal Enclosure System Rater Checklist states this:

3. Fully-Aligned Air Barriers.........At exterior surface of walls in all Climate Zones; and also at interior surface of walls for Climate Zones 4-8.......we are in 5.

So my question is, is the foundation wall a "Wall" or are they referring to above grade walls? I can't ask my Provider because they are worthless. I am switching Providers this year to a local outfit.

The poly is on the face of the stud. The FB is not right up against the concrete for obvious moisture concerns. I would like to at least align the pressure boundary (which would be the poly) with the insulation. Otherwise the conditioned air will bypass the FB if the only air barrier the concrete.

I'm trying really hard to help this guy, and not be the conceded building scientist that saying everything will fail. Especially given that he has some 500 in the ground spanning back to the 80's.

[Ted S.]

Bad building science accepted by the building authority--poly over fg in a basement--

Actually he said with the new energy codes he thinks everything will rot within a few years. He's actually a nice guy, I think he may accept Tyvek in the end. But I am trying to do my diligence.

There's a product called WallShield at 212 perms, maybe that will work.

[worthy]

Wallshield performs like a housewrap--as a membrane to protect the interior from the movement of liquid water from the exterior. Neither it, or Tyvek and its kin, make any sense in being used in the interior basement. They will do nothing to stop the movement of water vapour to the exterior during cold weather.

By contrast, MemBrain and any equivalent competitive products, reverse their permeability depending on the humidity. When there is low humidity during the winter, the permeability decreases, so water vapour from the interior doesn't enter the wall and vulnerable-to- mould fg insulation. In summer, when the vapour drive is from the exterior, the permeability increases so the wall can dry to the interior. MemBrain was tested in Minnesota and succeeded in those conditions that are similar to yours. And to mine in Toronto.

The FB is not right up against the concrete

Assuming that FB means fibreglass, all that means is that you've created a wonderful space for convective currents to completely negate any insulation value of the fiberglass. And it'll get wet anyway from the uninhibited vapour drives.

I'm no scientist. But with 500 units at stake, your client would be advised to invest a few hundred dollars for the advice of a qualified building scientist.

See IX from this informative Technical Bulletin from the Insulation Contractors Association of America .

(FWIW, I only insulate basements now with either ccspf or with 1" of XPS attached to the foundation wall followed by fg between the studs and no poly.)

[ThingOfBeauty]

what about perforated FSK (see page 15 of this document for a picture), or the poly replacement for it?
That's what's in most basements around here.
Doesn't do much for moisture (that's the idea) but does provide some semblance of air barrier and the FSK is UV resistant.

[Ted S.]

Thanks fellas.

[David Meiland]

Not that you asked, but I would hesitate to be the guy who did a bit of datalogging and then pronounced everything OK. A measurement like that is only a specific point in time. What if the landscaper accidentally drops another ten yards of topsoil against the wall, plants a bunch of shrubs, and installs irrigation... then the footing drains eventually clog up... then the adjacent property is built and a bunch of water is channeled against the wall... problems may happen.

I'm pretty sure that in your example, the pressure boundary is the concrete. I don't know if you can create one without a rigid material like drywall. How the TBC applies, not sure. I do know that you could call Washington State University Extension Energy Program and ask them, they answer those questions for most/all of the country... http://energy.wsu.edu/ dig around a bit, there is a phone number with a live person on it.

[Ted S.]

Not that you asked, but I would hesitate to be the guy who did a bit of datalogging and then pronounced everything OK.

You are probably right. Except I don't pronounce everything "OK" I just pronounce that one unit not particularly "fuc*ed".

I'm pretty sure that in your example, the pressure boundary is the concrete.

I know you are right there, every time we test out on a typically house, I throw the hose down the stairs and the primary air barrier is the foundation wall. Last one, the builder SPF'ed the rim joist, so the measurement from the house to the basement was 7 pa and from the basement to the exterior was 43 pa. But yet the "framed floor" is the thermal envelope. So the insulation is typically bypassed. It's silly, really.

But WTF I say, there are so many opinions out there I just get tired of it. If it really was building "science" everyone would have the same answer. I have a Provider saying one thing, internal Program QA/QC saying another thing, and external QA/QC saying something else, all comprised of individuals with no formal education in science or engineering, all disagreeing.

I should just throw down and say CCSPF is the only way to go, so then I would get fired. I stand to make $600-$1000 per unit, with a total of 250 more units to be built, I need that for my business. I can keep one guy going all the time on that.

I really don't want to cause a fuss, I know it's not the best detail in the world, but there must be a few million built with the same detail that are surviving. Including the 500 he built spanning back to the 80's.

[David Meiland]

I stand to make $600-$1000 per unit, with a total of 250 more units to be built

Ha, I can do those for $599-$999 apiece, and I can throw in the last one for free. What's the builder's name again?

[Ted S.]

Ha, I can do those for $599-$999 apiece, and I can throw in the last one for free. What's the builder's name again?

You just have to signoff on the stinker detail (fiberglass in the basement) ....